Nuclear fission is a piece of pre-(H)istory that we have large quantities of information on, but simply cannot replicate. Using pure mundane tools or magica enhanced ones, nothing works. It is possible that some fundamental laws in our world are either changed or restricted from when that information was first written down. It is also possible that the data that we have has all been subtlety changed. The "rule" that energy can neither be created or destroyed also does not seem to apply, as Magica is able to both generate and destroy matter and energy fairly easily. Some argue that it is accessing a different plane of reality and drawing energy or displacing energy to that plane, so the rule is still in effect, but that theory is not currently provable.
I do know there are rumors that the more powerful Walled Cities have been experimenting with weaponizing the meltdown state of a magica reactor, but down that path lies madness.
Much better to instead harness the meltdown state to the underside of a Walled City, propelling it up, hopefully with enough power to pierce the firmament of the heavens!
-Musings of the "Space Wizard" Nasarcanus